As known, numeric control milling machines with column structure are usually provided with a large base plate, which is structured to stably rest on and be anchored to the ground; with a movable supporting column that rises cantilevered from the base plate in a vertical direction and is capable of moving horizontally on the base plate along straight guides, which are parallel to the ground and to a first horizontal reference axis of the machine; and with a part-holding table, which is located by the side of the base plate and is structured so as to be able to firmly hold the metal part to be milled, usually also with the possibility of turning the metal part on command about one or more reference axes which are orthogonal to one another.
In the most sophisticated milling machines currently available on the market, the supporting column is moved along the straight guides of the base plate by means of a large electronically-controlled electric linear motor, which is interposed between the base plate and the lower end of the supporting column.
The numerical control milling machines with column structure are furthermore provided with a large head-carrier slide, which is inserted in vertically sliding manner in the supporting column so as to be able to move along the supporting column parallel to a second machine reference axis which is vertical and orthogonal to the first reference axis; and with a long straight supporting beam with square section, which is inserted in pass-through and axially sliding manner in the head-carrier slide, so as to cantilevered jut out from the supporting column towards the adjacent part-holding table, while remaining parallel to the ground and to a third machine reference axis which is horizontal and perpendicular to both the first and the second machine reference axes.
Finally, the numeric control milling machines with column structure are provided with an electronically-controlled drive system capable of horizontally moving the supporting beam forwards and backwards with respect to the head-carrier slide, so as to be able to vary the length of the supporting beam stretch that cantilevered juts out from the column towards the part-holding table; and with a spindle-carrier head which is fixed onto the distal end of the horizontal supporting beam so as to be able to reach the metal piece located on the part-holding table.
In the more sophisticated milling machines currently on the market, the head-carrier slide is moved along the supporting column by means of one, or more often, two recirculating ball screws that are arranged vertically on the supporting column, on opposite sides of the head-carrier slider, and are driven by a big electrically-operated geared motor.
Unfortunately, in the taller column milling machines, the length of the recirculating ball screws may reach values which are so high as to limit in significant manner the performance of the numerical control machine. In fact, the inertia of these particular metallic components does not allow to impress high value accelerations to the head-carrier slide.